Conventionally, a radio apparatus in a radio communication system includes an amplifier that amplifies power of a transmission signal. In the radio apparatus, in order to increase power efficiency of the amplifier, the amplifier is generally operated near a saturation region of the amplifier. However, if the amplifier is operated near the saturation region, nonlinear distortion of the amplifier increases. Therefore, in order to suppress the nonlinear distortion and decrease an adjacent channel leakage ratio (ACLR), the radio apparatus is provided with a distortion compensation apparatus that compensates nonlinear distortion.
There is a “predistortion method” as one of distortion compensation methods used in a distortion compensation apparatus. Hereinafter, “predistortion” may be referred to as “PD”. A distortion compensation apparatus of a PD method suppresses distortion of an output of an amplifier by multiplying beforehand a transmission signal before input to the amplifier by a distortion compensation coefficient having inverse characteristics of nonlinear distortion of the amplifier to increase the linearity of the output of the amplifier. A signal after multiplying the transmission signal by the distortion compensation coefficient may be referred to as “predistortion signal (PD signal)”. Accordingly, the PD signal is a signal distorted beforehand according to the inverse characteristics of the nonlinear distortion of the amplifier before being input to the amplifier.
For example, as the distortion compensation apparatus of a PD method, there is an apparatus that has a table in which a plurality of distortion compensation coefficients are stored and reads out a distortion compensation coefficient depending on the power of the transmission signal from the table. The distortion compensation coefficients stored in the table are sequentially updated so that an error between a transmission signal as a reference signal and a signal output from the amplifier and fed back (hereinafter, “feedback signal”) becomes minimum.
FIG. 1 is an explanatory diagram of the problem. If an amplifier is operated in a nonlinear region with respect to a transmission signal, an inter modulation distortion (hereinafter, “IM”) being nonlinear distortion may be generated. For example, as illustrated in FIG. 1, if a transmission signal with a central frequency Ftx is amplified in the nonlinear region, a signal output from the amplifier may include IM3 and IM5 that are third-order IM and fifth-order IM, respectively. The IM is generated at frequencies “Ftx±BW” and “Ftx±2BW” away from the central frequency Ftx by an integral multiple of a bandwidth BW of the transmission signal, as illustrated in FIG. 1. It is assumed here that a distortion compensation apparatus compensates the IM up to the IM5. That is, it is assumed that a frequency band subjected to distortion compensation (hereinafter, “distortion compensation band”) is a band that covers up to the IM5.
The amplifier amplifies the transmission signal after having been frequency-converted by a frequency converter, for example, by using a local signal corresponding to the central frequency Ftx of the transmission signal. In this case, a feedback signal from the amplifier is frequency-converted by the frequency converter by using the local signal corresponding to the central frequency Ftx of the transmission signal, analog-to-digital converted by an analog to digital converter (ADC), and input to the distortion compensation apparatus. A frequency band subjected to the analog-to-digital conversion by the ADC (hereinafter, “AD conversion band”) is generally set, matched with the “distortion compensation band” described above. However, because the “distortion compensation band” is set so that all the IMs are compensated, the bandwidth of a “distortion compensation band” becomes wider as a bandwidth BW of the transmission signal becomes wider, and thus the bandwidth of the “AD conversion band” also becomes wide equally to the “distortion compensation band”. For example, if the bandwidth of the “distortion compensation band” is as illustrated in FIG. 1, the bandwidth of the “AD conversion band” becomes “5BW”, which is five times the bandwidth BW of the transmission signal. Generally, as the bandwidth of the “AD conversion band” becomes wider, the price of the ADC increases. As a result, it becomes difficult to compensate nonlinear distortion of an amplifier by using an inexpensive ADC.